The invention relates to inductively-heated ovens or installations used for performing heat treatment, and in which the gas used in the treatment is preheated prior to being introduced into the treatment chamber of the oven. Such ovens are used in particular for performing thermochemical treatments such as carburizing parts or densifying porous substrates by chemical vapor infiltration.
The field of application of the invention is that of making parts out of composite material that is thermostructural, i.e. material that presents both mechanical properties making it suitable for constituting structural parts, and the ability to conserve those properties up to high temperatures. Typical examples of thermostructural composite materials are carbon/carbon (C/C) composites having a carbon fiber reinforcing texture that is densified by a pyrolytic carbon matrix, and ceramic matrix composites (CMCs) having a refractory fiber reinforcing texture (made of carbon or of ceramic) that is densified by a ceramic matrix.
A well-known process for densifying porous substrates to make C/C or CMC composite parts is chemical vapor infiltration (CVI). The substrates for densifying are placed in a loading zone of a furnace or oven where they are heated. A reactive gas containing one or more gaseous precursors for the material constituting the matrix is introduced into the oven. The temperature and the pressure inside the oven are controlled so as to enable the reactive gas to diffuse within the pores of the substrates and form therein a deposit of the matrix-constituting material by decomposing one or more constituents of the reactive gas or by chemical reaction between a plurality of constituents, where the constituents form the precursor of the matrix. The process is performed under low pressure so as to encourage diffusion of the reactive gas into the substrates. The temperature at which the precursor(s) are transformed so as to form the material of the matrix, such as pyrolytic carbon or a ceramic, is generally higher than 900° C., and is typically close to 1000° C.
In order to ensure that densification of the substrates takes place throughout the loading zone of the oven as uniformly as possible, both in terms of the increase in density and in terms of the microstructure of the matrix material that is formed, it is necessary for the temperature in the loading zone to be substantially uniform.
Thus, ovens usually include a preheater chamber or zone for preheating the reactive gas and situated between the inlet for reactive gas into the oven and the loading zone. Typically, the preheater zone comprises a plurality of perforated plates through which the reactive gas passes.
The plates for preheating the gas, like the substrates, are heated because of their presence in the oven. The oven itself is generally heated by means of an induction secondary known as a “susceptor”, e.g. made of graphite, that defines the side wall of the oven and that is coupled to a field winding or “induction coil” surrounding the oven. In accordance with the well-known principle of induction heating, when the susceptor is placed in a varying magnetic field that is generated by current flowing in the induction coil, an induced current flows in the susceptor, which induced current “reflects” the inducing current. The induced current flow in the susceptor causes it to be heated by the Joule effect. The heat as dissipated in this way is transmitted by radiation into the oven enclosure as defined by the susceptor.
With ovens of large dimensions (large diameters), the Applicant has observed that the loaded substrates are subject to substantial temperature variations. A significant example is that of isothermal chemical vapor infiltration (ICVI) of substrates, where the substrates are constituted by annular preforms of carbon fibers or by predensified annular blanks, for the purpose of making C/C composite brake disks. The substrates are placed in one or more vertical stacks in the loading zone, above the reactive gas preheater chamber that is situated in the bottom portion of the oven. It is important to minimize variations of the reactive gas temperature during densification to reduce densification non-uniformities and avoid generation of undesired species. Unfortunately, with a preheater chamber of the kind described above, substantial temperature variations have been observed.
In general, for any heat treatment oven having a gas preheater chamber, it is desirable for the preheating of the gas at the outlet from such a chamber to allow for an efficient thermal control all along the heat treatment.
In order to solve this problem, it might be envisaged that the effectiveness with which the gas is preheated might be increased by enlarging the heater zone, in particular by increasing its volume heightwise to the detriment of the volume of the loading zone, for given total oven volume. However, treatments such as the chemical vapor infiltration process requires a large amount of investment on an industrial scale, and the treatment can take a very long time to perform. It is therefore highly desirable for ovens to present high levels of productivity, regardless of whether the ovens are already in service or are new ovens to be made, and therefore it is desirable for the working volume dedicated to loading substrates or parts for treatment to be as great as possible in comparison with the volume dedicated to heating the reactive gas.